Commitments and Contingencies - Narrative (Details) $ in Thousands		1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Jul. 31, 2018 USD ($) petition	Nov. 30, 2017 company petition patent	May 31, 2016 patent	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) party	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Lessee, Lease, Description [Line Items]
Number of parties with patent license agreement | party						4
Royalty expense						$ 4,400	$ 1,400	$ 1,100
Percentage of precision oncology testing revenue						2.00%	2.00%	2.00%
Other Income
Lessee, Lease, Description [Line Items]
Amount awarded from other party					$ 4,250
Foundation Medicine, Inc. vs. Guardant Health, Inc..
Lessee, Lease, Description [Line Items]
Number of petitions filed | patent				3
Payments for legal settlements		$ 3,000
Foundation Medicine, Inc. vs. Guardant Health, Inc.. | General and administrative expense
Lessee, Lease, Description [Line Items]
Litigation settlement, expense	$ 3,000
Personal Genome Diagnostics, Inc. vs. Guardant Health, Inc.
Lessee, Lease, Description [Line Items]
Number of petitions filed | petition		6	2
Number of companies that infringed on a patent | company			2
Number of asserted patents | patent			2